In the prior art, filters which comprise unitary filter mats are predominantly used today for the filtration of transmission oil and engine oil. These are usually nonwoven mats, which may contain glass, cellulose or synthetic fibres and also metal fibres. With these filter media, contaminants of a size from >60 μm can be effectively removed from the liquid to be filtered. However, smaller particles usually cannot be adequately filtered with this material.
With the development of new generations of engines and transmissions, in particular continuously variable transmissions (CVTs), it has become necessary to provide filter materials and filters which ensure a higher purity of the transmission oil. This is associated with the fact that, in CVTs for example, the hydraulic control takes place electronically or else pressure-dependently, and consequently the smallest contaminants are enough to interfere with the transmission control. The classes of oil purity necessary for the operation of transmissions of this type can only be achieved at present by including an additional pressure oil filter. Applications for increasing the purity of engine oil are also possible.
In future, it is intended to be possible for the entire spectrum of dirt particles to be filtered in a filter by suction oil filtration, and for the necessary oil purity class to be achieved by the step alone. To ensure this, it is therefore urgently required to provide filter materials which are more efficient with respect to their retention of contaminants and can also retain contaminants of smaller size. For this purpose, the properties of the filter media must be improved, in particular with regard to pressure loss during filtration, filtration efficiency and dirt take-up capacity.
To solve the problems concerned, it is already known from the prior art to use multilayer filter materials. The aim with theses multilayer filter materials is what is known as serial filtration, in which the coarse particles are initially separated out by means of one or more filter media and the fine particles are subsequently separated out by means of one or more other filter media. Composite filters of this type are currently already being used in transmissions, in particular on the American market. These are composite filter media which comprises a nonwoven filter mat and a woven filter fabric material, which are thermally bonded by means of an adhesive. Open-woven polyester adhesives, which are laid in the form of coarse-meshed gauzes with irregularly distributed meshes between the nonwoven filter mat and the woven filter fabric, are often used as the adhesive. Adhesive in granular form, which are sprinkled onto the materials, are also used. After that, the filter layers are adhesively bonded by heating.
However, these composite fibers have the disadvantage that, on account of the adhesive used and the full surface-area bonding, there is a considerable reduction in the filtration capability of the filter medium, since the adhesive closes the open pores of the woven filter fabric to the greatest extent and consequently greatly reduces the effective filtration surface area. Furthermore, the adhesive gauzes used have an entirely random gauze structure, which leads to irregular adhesive bonding. Consequently, some points in the filter medium have a better filtration capability than other points, at which for example higher adhesive concentrations are present. This results in composite filters with very different properties with regard to pressure loss, filtration efficiency and dirt take-up capacity, so that filter production with standardized properties is not possible. Rather, considerable quality fluctuations occur during production and the production of filters of this type is extremely expensive.
A further disadvantage is the durability of the adhesive bond. Filter media are exposed to considerable temperature fluctuations, which in an extreme case can lie between −30 and +160° C. The strength of the adhesive bond is clearly put under strain at high temperatures and at low temperatures. For example, the strength of the adhesive bond is considerably reduced in particular at high temperatures, so that a partial detachment of the filter layers from one another often occurs at these temperatures.
This has also had the effect that composite filters have not become established in this area in the past, with unitary filter mats being used instead as filter media in the majority of cases.
Furthermore, it is attempted in the prior art to achieve improvements in the filtration by pleating the filter medium. Filter sheet material folded in zigzag form is very often used here, in particular to increase the surface area of the filter media while keeping the spatial volume as small as possible. Filters of this type, in particular for engine oil and transmission oil, are described for example in DE 19735993 A1, WO 01/45823 A1 and DE 4227744 A1. In these cases, zig-zag-shaped filter media are used both in the oil filter area and in the air filter area.